Current ultrasonic imaging systems make use of contrast agents in circulation to enhance ultrasound returns. Contrast agents are substances which strongly interact with ultrasound waves and return echoes which may be clearly distinguished from those returned by blood and tissue. Microbubbles are currently employed as a contrast agent and provide a non-linear behavior in certain acoustic fields. Microbubble contrast agents are useful for imaging of the body's vascular system and are injectable through the veins and arteries. They are subsequently filtered from the blood stream by the lungs, kidneys and liver.
Microbubble contrast agents generally comprise coated gas bubbles that are stable in the body for a significant period of time. The coating shells serve to protect the gas from diffusion into the blood stream. At moderately high ultrasound pressure amplitudes, the shells of the microbubbles can be caused to rupture, freeing the internal gas and substantially reducing the detectability thereof by incident ultrasound waves.
U.S. Pat. No. 5,833,613 to Averkiou et al. discloses an ultrasound method for imaging of contrast agents. In one embodiment, a rate of re-perfusion of an anatomical region is accomplished by initially destroying the contrast agent within the region, and then subsequently imaging the region to determine the rate of re-insertion of the contrast agent.
U.S. Pat. No. 5,546,257 to Johnson et al. describe an ultrasonic system that detects and destroys microbubble contrast agent in circulation. The microbubble destruction is detected by phase insensitive detection and differentiation of echoes received from two consecutive ultrasonic transmissions.
When a region containing contrast agent is irradiated with a high energy ultrasound beam, the destruction of the contrast agent results in relatively high amplitude echo signals being returned to the receiving chain of the ultrasound unit. However, during such action, the echo returns from wall motion may be of an equal or higher amplitude. Such wall motion signals can mask or otherwise impair the signal from the contrast agent.
The prior art has utilized adaptive clutter filtering to determine the velocity of highest power echo signals, usually tissue. The adaptive clutter filter is then controlled so as to center its maximum attenuation at the determined velocity (from Doppler processing) so as to suppress tissue wall motion. The leftover signals are, in the main, those returned from blood flow.
This approach works well for conventional color flow imaging (CFMI) because signals from wall motion are often significantly higher in power than any other color flow signal of interest. Thus, the control processor could be assured of properly adjusting the frequency attenuation characteristic of the adaptive clutter filter if it set the filter to track the frequency of largest amplitude echo signals.
However, contrast agents, when destroyed provide a high power echo signal that exhibits chaotic phase characteristics. Thus when a standard adaptive clutter filter is caused to lock onto the signal echoes from the microbubble destruction due to their high energy, such action can cause the filter to place maximum attenuation on the microbubble destruction signal frequencies rather than the wall motion that is sought to be attenuated.
Accordingly, There is a need for an improved method for displaying to the user an image that evidences enhanced characteristics from microbubble contrast agent destruction. Further, a method is required that efficiently removes tissue wall motion echo returns so as to enable improved contrast agent display specificity